\(\begin{array}{l}
{m_1}{v_1}\hat i + {m_2}{v_2}\hat i = {m_1}{v_3}\hat i + {m_2}{v_4}\hat i\\
{m_1}{v_1} + 0.5\,{m_1}{v_2} = {m_1}\left( {0.5{v_1}} \right) + 0.5\,{m_1}\,{v_4}\\
0.5\,{m_1}{v_1} = 0.5\,{m_1}\left( {{v_4} - {v_2}} \right)\\
{v_1} = {v_4} - {v_2}
\end{array}\)